Chalk used to make markings on blackboards or chalkboards is typically made from calcium carbonate formed into a hard solid stick or cake. When the chalk is used, calcium carbonate is transferred to the blackboard surface. A large amount of calcium carbonate dust is also generated when the markings are made and when the blackboard erasers used to wipe away the markings are cleaned by slapping them together. This dust is more than just a housekeeping nuisance, it poses health concerns, especially in a classroom setting, because the inhalation of the dust may cause, inter alia, respiratory problems. In addition, the dust is harmful to electronic equipment, such as computers. Therefore, it is desirable to produce a marking medium useful on a standard blackboard or chalkboard which does not pose health and safety concerns.
Efforts have been made to produce marking media, such as, for example, liquid marking compositions, which do not generate dust or have other deleterious health effects. There has only been limited success in developing liquid marking compositions because, in the effort to produce a healthier marking medium, performance properties of the compositions have been compromised.
One problem is that the liquid marking compositions do not have the same whiteness or opacity on conventional blackboards as conventional chalk. Opacifiers have been added to solve this problem. As a result of the addition of the opacifying agents, however, many of the compositions lose their "erasability," i.e., they leave a hazy, whitish film on the dark blackboard or chalkboard after the board has been wiped with a conventional blackboard eraser or additionally wiped with water. The formation of this undesirable film is referred to as "ghosting."
Another problem with liquid marking compositions has been burnishing of the blackboard, i.e., a glazing, not associated with any physical abrasion, of the blackboard surface caused by a component of the liquid marking composition, such as soft, low melting point polymers and waxy materials.